# Discrete signals notation underlined value

In a book i have there's an exercise that want to find the output of a system for a given signal

system: $2y(n)=2x(n)+x(n+1)+x(n-1)$

input : $Χ=[\underline1, -1, 2, 0, -2, 0, 1 ]$

$Χ1=Χ[0]$

What represent's the underlined value?

• Just out of curiosity: which book are you referring to? (because I've never seen this notation) – Matt L. Jun 18 '14 at 8:46
• it's an internal book from my uni. – Giannis Foulidis Jun 18 '14 at 20:00
• Very interesting concept. – jojek Jun 28 '14 at 18:52

An arrow or an underline is used to indicate the value at $n=0$. Example,
$$y(n) = \begin{array}{ccccc}[&9 &8 &\underline7 &6 &5 &]\end{array}$$ means that, $$\begin{array}{ccccc}&y(-2) = 9, &y(-1) =8, &y(0)=7, &y(1) = 6 &\mathrm{and} &y(2) =5\end{array}$$
In your case, it says that $$\begin{array}{ccccc}&X(0) = 1, &X(1) =-1, &X(2)=2, &\ldots\end{array}$$